Method to modulate the sensitivity of a bolometer via negative interference

ABSTRACT

A semiconductor sensor system, in particular a bolometer, includes a substrate, an electrode supported by the substrate, an absorber spaced apart from the substrate, a voltage source, and a current source. The electrode can include a mirror, or the system may include a mirror separate from the electrode. Radiation absorption efficiency of the absorber is based on a minimum gap distance between the absorber and mirror. The current source applies a DC current across the absorber structure to produce a signal indicative of radiation absorbed by the absorber structure. The voltage source powers the electrode to produce a modulated electrostatic field acting on the absorber to modulate the minimum gap distance. The electrostatic field includes a DC component to adjust the absorption efficiency, and an AC component that cyclically drives the absorber to negatively interfere with noise in the signal.

RELATED APPLICATION

This application is a 35 U.S.C. § 371 National Stage Application ofPCT/US2015/054855, entitled “METHOD TO MODULATE THE SENSITIVITY OF ABOLOMETER VIA NEGATIVE INTERFERENCE” by Rocznik et al., filed Oct. 9,2015, which claims the benefit of priority to U.S. ProvisionalApplication No. 62/062,436 filed on Oct. 10, 2014, entitled “METHOD TOMODULATE THE SENSITIVITY OF A BOLOMETER VIA NEGATIVE INTERFERENCE,” thedisclosures of which are incorporated by reference herein in theirentirety.

TECHNICAL FIELD

This disclosure relates generally to sensors and, more particularly, tobolometers.

BACKGROUND

Objects at any non-zero temperature radiate electromagnetic energy whichcan be described either as electromagnetic waves or photons, accordingto the laws known as Planck's law of radiation, the Stefan-BoltzmannLaw, and Wien's displacement law. Wien's displacement law states thatthe wavelength at which an object radiates the most (λmax) is inverselyproportional to the temperature of the object as approximated by thefollowing equation:

${\lambda_{\max}({\mu m})} \approx \frac{3000}{T(K)}$

Hence for objects having a temperature close to room temperature, mostof the emitted electromagnetic radiation lies within in the infraredregion. Due to the presence of CO₂, H₂O, and other gasses and materials,the earth's atmosphere absorbs electromagnetic radiation havingparticular wavelengths. Measurements have shown, however, that there are“atmospheric windows” where such absorption is minimal. An example ofsuch a “window” is the 8 μm-12 μm wavelength range. Another windowoccurs at the wavelength range of 3 μm-5 μm. Typically, objects having atemperature close to room temperature emit radiation close to 10 μm inwavelength. Therefore, electromagnetic radiation emitted by objectsclose to room temperature is only minimally absorbed by the earth'satmosphere. Accordingly, detection of the presence of objects which areeither warmer or cooler than ambient room temperature is readilyaccomplished by using a detector capable of measuring electromagneticradiation emitted by such objects.

One commonly used application of electromagnetic radiation detectors isfor automatically energizing garage door lights when a person or carapproaches. Another application is thermal imaging. In thermal imaging,which may be used in night-vision systems for driver assistance, theelectromagnetic radiation coming from a scene is focused onto an arrayof detectors. Thermal imaging is distinct from techniques which usephotomultipliers to amplify any amount of existing faint visible light,or which use near infrared (˜1 μm wavelength) illumination andnear-infrared cameras.

Two types of electromagnetic radiation detectors are “photon detectors”and “thermal detectors”. Photon detectors detect incident photons byusing the energy of said photons to excite charge carriers in amaterial. The excitation of the material is then detectedelectronically. Thermal detectors also detect photons. Thermaldetectors, however, use the energy of said photons to increase thetemperature of a component. By measuring the change in temperature, theintensity of the photons producing the change in temperature can bedetermined.

In thermal detectors, the temperature change caused by incoming photonscan be measured using temperature-dependent resistors (thermistors), thepyroelectric effect, the thermoelectric effect, gas expansion, and otherapproaches. One advantage of thermal detectors, particularly for longwavelength infrared detection, is that, unlike photon detectors, thermaldetectors do not require cryogenic cooling in order to realize anacceptable level of performance.

One type of thermal sensor is known as a “bolometer.” Even though theetymology of the word “Bolometer” covers any device used to measureradiation, bolometers are generally understood to be to thermaldetectors which rely on a thermistor to detect radiation in the longwavelength infrared window (8 μm-12 μm) or mid-wavelength infraredwindow (3 μm-5 μm).

Because bolometers must first absorb incident electromagnetic radiationto induce a change in temperature, the efficiency of the absorber in abolometer relates to the sensitivity and accuracy of the bolometer.Ideally, absorption as close to 100% of incident electromagneticradiation is desired. In theory, a metal film having a sheet resistance(in Ohms per square meter) equal to the characteristic impedance of freespace, laying over a dielectric or vacuum gap of optical thickness dwill have an absorption coefficient of 100% for electromagneticradiation of wavelength 4d. The following equation shows the expressionof the characteristic impedance (Y) of free space:

$Y = \sqrt{\frac{\mu_{0}}{ɛ_{0}}}$wherein ε₀ is the vacuum permittivity and μ₀ is the vacuum permeability.

The numerical value of the characteristic impedance of free space isclose to 377 Ohm. The optical height of the gap is defined as “n×d”,where n is the index of refraction of the dielectric, air or vacuum inthe gap.

In the past, micro-electromechanical systems (MEMS) have proven to beeffective solutions in various applications due to the sensitivity,spatial and temporal resolutions, and lower power requirements exhibitedby MEMS devices. One such application is as a bolometer. Knownbolometers use a supporting material which serves as an absorber and asa mechanical support. Typically, the support material is siliconnitride. A thermally sensitive film is formed on the absorber to be usedas a thermistor. The absorber structure with the attached thermistor isanchored to a substrate through suspension legs having high thermalresistance in order for the incident electromagnetic radiation toproduce a large increase of temperature on the sensor.

A temperature change of an absorber of a bolometer due to absorption ofincident radiation is associated with a change in resistance of athermistors material of the absorber. By measuring an output voltageresulting from applying a probe current across the absorber, the changein resistance in the absorber is determined. Using the correspondencebetween the change in resistance and the change in temperature of theabsorber, the change in resistance of the absorber is used to make aninference about the incident radiation.

The output voltage is a combination of signals corresponding to thetemperature change as well as offset, drift, and noise components. Thenoise component can include flicker (“1/f”) noise and thermal noise.FIGS. 1A-1C illustrates a conventional method of compensating for signalnoise components. FIG. 1A is a graph illustrates that a signal 12corresponding to an output voltage resulting from a probe currentcomprises noise components including 1/f noise 14 and thermal noise 16.Generally, such noise is compensated for by modulating the signal 12 toa higher frequency band, subjecting the modulated signal to a high-passfilter 18, as illustrated in the graph of FIG. 1B, and by returning thesignal 20 back to a base band through demodulation as illustrated in thegraph of FIG. 1C. As illustrated in FIGS. 1A-C, a signal-to-noise ratio(“SNR”) increases as a result of the modulation and demodulation scheme.

However if noise, such as flicker noise or low frequency drift noise, isembedded in the bolometer, conventional modulation is insufficient tocompensate for the noise components. What is needed, therefore, is amethod of modulating a bolometer that can compensate for a noise sourceembedded in the bolometer.

SUMMARY

In order to facilitate detection of radiation, in particular radiationin a long wavelength infrared window (8 μm-12 μm) and a mid-wavelengthinfrared window (3 μm-5 μm), a semiconductor sensor system includes anelectrode supported the substrate, an absorber structure suspended overand spaced apart from the electrode, a voltage source operativelycoupled to the electrode and the absorber structure, and a currentsource operatively coupled to the absorber structure.

The absorber structure includes an absorbing material and a thermistor.The electrode may operate as a mirror that reflects radiation backtoward the absorber structure. The sensor system may also include amirror separate from the electrode that can positioned between theelectrode and the substrate, above the absorber structure, next to theelectrode, or any other acceptable position. The mirror reflectsincident radiation back toward to absorber structure to be absorbed bythe absorber material.

The absorber material's ability to absorb radiation of differentwavelengths, i.e., the absorber material's sensitivity, is based atleast in part upon a height of a gap between the absorber structure andthe mirror, where the height of the gap is associated with a wavelengthof radiation to be detected by the system.

A temperature of the absorbing material changes due to the absorption ofradiation by the absorbing material. The thermistor is thermally coupledto the absorber material so that a temperature of the thermistor changesalong with the temperature of the absorber material. The thermistor hasa resistance that varies along with the temperature of the thermistor.

The current source applies a probe current across the absorberstructure, in particular across the thermistor, to enable a detection ofa change in the resistance of the thermistor that may be used as adetection signal to make an inference about the radiation absorbed bythe absorbing material.

The voltage source is operatively coupled to the electrode, and isconfigured to supply the electrode with power to produce a modulatedelectric potential that produces an electrostatic force that acts on theabsorber structure to modulate a minimum height of the gap.

The electric potential has a DC component that adjusts the height of thegap to be associated with a wavelength of radiation to be detected bythe system. The electric potential also has an AC component that drivesthe absorber structure with a carrier frequency. The carrier frequencymay be based at least in part upon a thermal time constant of theabsorber structure as well as a mechanical transfer function for theresiliency of the absorber structure that counteracts the action of theelectric potential.

The cyclical repositioning of the absorber structure due to the carrierfrequency results in a cyclical adjustment of the height of the gap anda corresponding cyclical adjustment of the absorption efficiency of theabsorber material, which introduces an AC component into the detectionsignal. The AC component in the detection signal negatively interfereswith noise embedded in the sensor device, such as thermal noise andflicker noise.

In one embodiment, the sensor system includes a second absorberstructure and a second electrode that form a differential pair with theabsorber structure and electrode. The voltage source supplies the secondelectrode with power to produce a second electric field. However, thesensor system in this embodiment further includes an inverter that phaseshifts the AC component by 180 degrees relative to the AC component ofthe electric potential of the electrode. A potential difference indetection signals from the first absorber structure and second absorberstructure forms a differential detection signal of the sensor system.

A filter, such as a high pass filter, may be used to isolate componentsof the detection signal that are indicative of the radiation absorbed bythe absorber material and to filter out low frequency noise and thermaldrift. The filter may be based on the carrier frequency of the ACcomponent of the electric potential, as well as the mechanical transferfunction of the absorber structure. The filtered detection signal maythen be brought back to base band frequency through demodulation.

In an embodiment, a free standing bolometer includes an absorber and amirror/electrode, and is configured to be electrostatically actuated.

In one embodiment, the mirror/electrode is below the absorber. Inanother embodiment, the mirror/electrode is above the absorber.

In a further embodiment, a signal modulation scheme for a bolometercomprises gap variation between an absorber and a mirror/electrode,wherein the gap variation is configured to cause constructive ordestructive interference, for example, in order to compensate for noisesuch as flicker noise and thermal noise.

In another embodiment, a bolometer includes a sensor, wherein amirror/electrode of the sensor is supplied with an AC voltage overlaidwith a DC component.

In an additional embodiment, a bolometer includes a first sensor and asecond sensor that form a differential pair, wherein a mirror/electrodeof the first sensor is supplied with an AC voltage overlaid with a DCcomponent that is phase shifted by 180 degrees from an AC voltageoverlaid with a DC component that is supplied to the second sensor.

In an embodiment, a micro-electromechanical systems (MEMS) bolometersystem includes a substrate, a first absorber structure, a firstelectrode, and a voltage source. The first absorber structure is spacedapart from the substrate by a first gap. The first electrode issupported by the substrate and is spaced apart from the first absorber.The voltage source is operatively coupled to the first electrode and isconfigured to generate a first modulated electrostatic force on thefirst absorber structure using the first electrode such that a minimumheight of the first gap above the substrate is modulated by the firstmodulated electrostatic force.

In one embodiment, a first modulated electrostatic force generated by avoltage source of a MEMS bolometer system includes a DC component and anAC component.

In another embodiment, a MEMS bolometer system includes a mirrorsupported by a substrate at a location aligned with at least a portionof a first absorber structure.

In a further embodiment, a mirror of a MEMS bolometer system is locatedbetween a first electrode and at least a portion of a first absorberstructure.

In an embodiment, a first electrode of a MEMS bolometer system includesa mirror supported by a substrate at a location aligned with at least aportion of a first absorber structure.

In one embodiment, a MEMS bolometer system includes a second absorber, asecond electrode, and an AC inverter. The second absorber structure isspaced apart from the substrate by a second gap. The second electrode issupported by the substrate and is spaced apart from the second absorber.The AC inverter has an output operatively coupled with the secondelectrode, and is configured to generate a second modulatedelectrostatic force on the second absorber structure using the secondelectrode such that a minimum height of the second gap is modulated bythe second modulated electrostatic force. A phase of a first modulatedelectrostatic force of the MEMS bolometer system is shifted by 180degrees from a phase of the second modulated electrostatic force.

In another embodiment, a first absorber structure of a MEMS bolometersystem has a mechanical reaction time and a thermal time constant, andan AC component of an electrostatic force generated by a voltage sourceof the MEMS bolometer system has an AC component. A maximum frequency ofthe AC component is based upon the mechanical reaction time and thethermal time constant.

In a further embodiment, a MEMS bolometer system includes a high passfilter operatively connected to an absorber structure, and a demodulatoroperatively connected to an output of the high pass filter.

In an additional embodiment, a method of operating a MEMS bolometersystem includes spacing a first absorber structure apart from asubstrate by a first gap, generating a first modulated electrostaticforce on the first absorber structure using a first electrode supportedby the substrate and spaced apart from the first absorber structure, andmodulating a minimum height of the first gap above the substrate usingthe first modulated electrostatic force.

In an embodiment, a method of operating a MEMS bolometer system includesgenerating a first modulated electrostatic force using a DC componentand an AC component.

In one embodiment of a method of operating a MEMS bolometer system,spacing a first absorber structure apart from a substrate by a first gapincludes selecting a distance associated with a local maximum or minimumof a mean normalized absorption of the first absorber structure for awavelength of interest, and spacing the first absorber structure apartfrom the substrate by the selected distance.

In another embodiment of a method of operating a MEMS bolometer system,modulating a minimum height of a first gap between an absorber structureand a substrate includes using a DC component of an electrostatic forceto bias the first absorber structure from a first location associatedwith a selected distance to a second location closer to the substratesuch that a first variation of the mean normalized absorption of thefirst absorber structure for a given change in the minimum height of thefirst gap at the second location is greater than a second variation ofthe mean normalized absorption of the first absorber structure for thegiven change in the minimum height of the first gap at the firstlocation.

In a further embodiment of a method of operating a MEMS bolometersystem, spacing a first absorber structure apart from a substrate by afirst gap includes spacing the first absorber structure apart from amirror supported by the substrate at a location aligned with the mirror.

In an additional embodiment, a method of operating a MEMS bolometersystem includes positioning a mirror at a location between a firstelectrode and at least a portion of a first absorber structure.

In a further embodiment of a method of operating a MEMS bolometersystem, a first electrode includes a mirror supported by a substrate ata location aligned with at least a portion of the first absorberstructure.

In an additional embodiment, a method of operating a MEMS bolometersystem includes spacing a second absorber structure apart from asubstrate by a second gap, generating a second modulated electrostaticforce on the second absorber structure using a second electrodesupported by the substrate and spaced apart from the second absorberstructure, the generating using the DC component and a further ACcomponent that is phase shifted by 180 degrees relative to an ACcomponent of a first modulated electrostatic force, and modulating aminimum height of the second gap above the substrate using the secondmodulated electrostatic force.

In one embodiment, a method of operating a MEMS bolometer systemincludes isolating a signal indicative of radiation absorbed by a firstabsorber structure from a voltage in a first absorber structure via ahigh pass filter operatively connected to the absorber structure, anddemodulating the signal via a demodulator operatively connected to anoutput of the filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are graphs that illustrate a known modulation schemeincluding a noise component for filtering noise from a signal.

FIG. 2 depicts a side view of a bolometer with an absorber that providesthe function of a thermistor in accordance with principles of thisdisclosure.

FIG. 3A depicts a side plan view of the bolometer of FIG. 2 along with apartial electrical schematic of the bolometer.

FIG. 3B depicts an electrical schematic of the bolometer of FIG. 2.

FIG. 4 depicts a side plan view of the bolometer depicted in FIG. 2,wherein the absorber is in a modulated position.

FIG. 5 is a graph of simulation data illustrative of absorption vs. avarying gap between the absorber and the mirror in the bolometerdepicted in FIG. 2.

FIG. 6 depicts an electronic schematic of a multi-sensor bolometeraccording to this disclosure.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of theembodiments described herein, reference is now made to the drawings anddescriptions in the following written specification. No limitation tothe scope of the subject matter is intended by the references. Thisdisclosure also includes any alterations and modifications to theillustrated embodiments and includes further applications of theprinciples of the described embodiments as would normally occur to oneskilled in the art to which this document pertains.

FIG. 2 depicts a side plan view of a semiconductor sensor 100 which inthis embodiment is a bolometer. The sensor 100 includes a substrate 102,a mirror, 104 an absorber 106, and suspension legs 108 and 110.

The substrate 102 may be a complementary metal oxide semiconductor(CMOS) substrate or any other acceptable type of substrate. In thisembodiment, the substrate 102 is a silicon wafer. While FIG. 2illustrates only one sensor 100 formed on the substrate 102, thesubstrate 102 may include any acceptable number of sensors 100, and mayinclude electronic circuitry usable to access an output of the sensor100.

The mirror 104 is disposed on the substrate 102, and may be, forexample, a metal reflector or a multilayer dielectric reflector. Theabsorber 106 is suspended over the mirror 104 by suspension legs 108 and110 to form a gap between the absorber 106 and mirror 104. In thisembodiment, the minimum height of the gap between the mirror 104 and theabsorber 106 is about 2.5 μm. Since the efficiency of the absorber 106for absorbing different wavelengths of radiation is related to the gap,the gap is selected to optimize absorption in a wavelength region to besensed. In this embodiment, the 2.5 μm height of the gap is associatedwith the long-wavelength infrared region.

The absorber 106, in addition to absorbing energy from incident photons,is selected to provide a good noise-equivalent temperature difference(NETD). In order for the absorber 106 to have a good NETD, the materialselected to form the absorber 106 should exhibit a high temperaturecoefficient of resistance while exhibiting low excess noise (1/f noise).Semiconductor materials such as vanadium oxide are common inmicro-machined bolometers due to their high temperature coefficient ofresistance. While metals have a lower temperature coefficient ofresistance than some semiconductor materials, such as vanadium oxide,metals typically have much lower excess noise than many semiconductormaterials.

Accordingly, in this embodiment the absorber 106 comprises metal.Titanium and Platinum are two metals which exhibit desiredcharacteristics. Titanium, for example, exhibits a bulk resistivity ofabout 7*10-7 Ohm. Using a bulk resistivity of 7*10-7 Ohm, the thicknessof the absorber 106 to match the impedance of free-space Y (377Ohm/square meter) should be about 1.9 nm. The resistivity of materialsformed to a thickness less than about 50 nm, however, can be severaltimes higher than the bulk value. Accordingly, depending on processparameters, the thickness of the absorber 106, if made from titanium, ispreferably about 10 nm. Impurities can also be introduced into theabsorber 106 during formation in order to tune the resistivity ifneeded.

Consequently, the thickness of the absorber 106 in this embodiment isabout 10 nm and the length of the absorber 106 from the suspension leg108 to the suspension leg 110 is about 25 μm. This configurationprovides a ratio between the thickness of the absorber 106 and thelength of the absorber 106 in the order of 1/1000 and the ratio of thethickness of the absorber 106 to the gap height of about 1/100.

Other aspects of a bolometer device such as the embodiment illustratedin FIG. 2 are described in U.S. Pat. No. 7,842,533, granted Nov. 30,2010, the disclosure of which is incorporated herein by reference in itsentirety. Where a definition or use of a term in a reference isinconsistent or contrary to the definition of that term provided herein,the definition of that term provided herein applies and the definitionof that term in the reference does not apply.

In the embodiment illustrated in FIGS. 2-4, the mirror 104 isadditionally configured as an electrode, although in some embodiments, aseparate electrode is provided, either above or below the mirror 104.

As illustrated in FIGS. 3A and 3B, the electronic schematic 300 of thesensor 100 includes a DC current source 112, and an output voltage 114.The DC current source 112 supplies the absorber 106 with a current, inparticular, a probe current. The output voltage 114 reflects aresistance change in the absorber 106 due to absorption of incidentradiation, and thus corresponds to a detector signal of the sensor 100.

The detector signal of the sensor 100 is modulated by themirror/electrode 104 in order to compensate for noise componentsembedded in the sensor 100 using the electronic schematic 300. Themirror/electrode 104 is configured to create an electric potentialbetween the electrode 104 and the absorber 106 which exerts anelectrostatic force that repositions the absorber 106. A voltage source116 powers the mirror/electrode 104, and thus drives the electricpotential. The voltage source 116 has a DC component configured tomodulate or modify a position of the absorber 106 and an AC componentconfigured to drive the absorber 106 with a carrier frequency.

FIG. 4 depicts a side plan view of the sensor 100, in which the voltagesource 116 is powering the mirror/electrode 104 with the DC component.The DC component drives an electric potential between themirror/electrode 104 and the absorber 106. The electric potential exertsan electrostatic force that repositions the absorber 106 such that theabsorber 106 is in a modified position relative to a rest position 118.In other words, the electrostatic force acts on the absorber 106 tochange a minimum distance of a gap 120 between the absorber 106 and themirror/electrode 104. The rest position 118 corresponds to a position ofthe absorber 106 when the DC component of the voltage source 116 is 0 V.

While the electrostatic force acts on the absorber 106, a spring force,i.e. a resiliency, of the absorber 106 counteracts the electrostaticforce, such that actuation of the absorber 106 is based upon theelectrostatic force and a mechanical transfer function of a structure ofthe absorber 106 and the mirror/electrode 104. In other words, an extentto which the absorber 106 is repositioned is based at least in part uponthe voltage source 116 that powers the mirror/electrode 104, but alsoupon mechanical properties of the absorber 106, the mirror/electrode104, and other structure of the sensor 100.

For different minimum heights of the gap 120, the sensor 100 exhibitsdifferent absorption efficiencies for various wavelengths of radiation.Thus, an absorption efficiency of the absorber 106 is modulated bymodifying the minimum distance of the gap 120 using the mirror/electrode104.

FIG. 5 illustrates a graph 502 of simulation data of a mean normalizedabsorption of radiation by the absorber 106 due to variation of the gap120. The rest position 118 is selected to correspond to a point on thegraph 502 that is close to a local maximum or minimum of the meannormalized absorption. In one embodiment, the minimum distance betweenthe mirror and the absorber in the rest position is about 5 μm,corresponding to a local minimum 504, or, in another embodiment, theminimum distance between the mirror and the absorber in the restposition is about 7 μm, corresponding to a local maximum 506.

When the rest position 118 is at a point on the graph close to a localminimum or maximum, the DC component of the voltage source 116 modulatesthe absorption efficiency of the absorber 106 to a point on the graph502 having a higher slope relative to a local minimum or maximum. In oneembodiment, the DC component modulates a minimum height of the gap 120from about 7 μm, corresponding to the local maximum 506, to a height ofabout 6 μm corresponding to the point 508. In another embodiment, the DCcomponent modulates a minimum height of the gap 120 from about 5 μm,corresponding to the local minimum 504, to a height of about 4 μmcorresponding to the point 510.

Because the points 508 and 510 on the graph 502 have a high sloperelative to a local minimum or maximum, the carrier frequency of the ACcomponent, when overlaid on the DC component, will have a greater effecton the absorption efficiency of the absorber 106. The output signalresulting from the probe current of the DC current source 112 willexhibit an AC component resulting from the AC component of the voltagesource 116. A maximum actuation frequency for the carrier frequencydepends at least in part upon a thermal time constant of the absorber106 and a mechanical reaction time of the absorber 106. The resultingoutput of the sensor 100 can be filtered using a high pass filter in amanner similar to the modulation described with regard to the middlegraph of FIGS. 1A-C above.

FIG. 6 depicts a circuit diagram of a sensor system 600 which, in thisembodiment is a multi-sensor bolometer. The system 600 includes aplurality of sensors, in this embodiment a first sensor 602 and a secondsensor 604, a DC current source 606, a voltage source 608, an inverter610, and a filter circuit 614, where each sensor 602 and 604 includes anabsorber structure and electrode.

The DC current source 606 supplies the first sensor 602 and the secondsensor 604 with a DC current, in particular, a probe current. Thevoltage source 608 supplies each of the first sensor 602 and the secondsensor 604 with a DC current component, supplies the first sensor 602with a first AC current component, and supplies the second sensor 604with a second AC current component that is phase shifted 180 degrees bythe inverter 610.

Outputs of the first sensor 602 and second sensor 604 are compared toform an output voltage 612. The first sensor 602 and the second sensor604 thus form a differential pair. The output voltage 612 is modulatedby a carrier frequency of the voltage source 608 and modified bymechanical transfer functions of the first sensor 602 and the secondsensor 604.

The filter circuit 614 acts on the output voltage, and includes a highpass filter and a demodulator, which operate in a manner similar to themodulation described with regard to the middle graph of FIGS. 1A-C aboveto isolate components of the detection signal that are indicative of theradiation absorbed by the absorber material and to filter out lowfrequency noise and thermal drift and return the resulting filteredsignal back to base band frequency through demodulation.

While the above embodiments have been described with reference todetection of infrared radiation using a bolometer, the reader shouldappreciate that the above-described tool is not limited to infraredradiation. A bolometer according to the present disclosure can beconfigured for other types of detection, for example detectingparticles, gravity waves, microwaves, and far-infrared radiation.Additionally, while various embodiments have been described above, thepresent disclosure is not limited to such embodiments. Other embodimentsinclude one or more features described above.

It will be appreciated that variants of the above-described and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems, applications or methods.Various presently unforeseen or unanticipated alternatives,modifications, variations or improvements may be subsequently made bythose skilled in the art that are also intended to be encompassed by theforegoing disclosure.

The invention claimed is:
 1. A micro-electromechanical systems (MEMS)bolometer system, comprising: a substrate; a first absorber structurespaced apart from the substrate by a first gap; a first electrodesupported by the substrate and spaced apart from the first absorberstructure; and a voltage source with a DC component and an AC component,the AC component having a frequency selected to compensate for noiseembedded within the system, the voltage source operatively coupled tothe first electrode and configured to generate a first modulatedelectrostatic force on the first absorber structure using the firstelectrode such that a minimum height of the first gap above thesubstrate is modulated by the first modulated electrostatic force. 2.The MEMS bolometer system of claim 1, further comprising: a mirrorsupported by the substrate at a location aligned with at least a portionof the first absorber structure.
 3. The MEMS bolometer system of claim2, wherein the mirror is located between the first electrode and the atleast a portion of the first absorber structure.
 4. The MEMS bolometersystem of claim 1, wherein the first electrode comprises a mirrorsupported by the substrate at a location aligned with at least a portionof the first absorber structure.
 5. The MEMS bolometer system of claim1, wherein: the first absorber structure has a mechanical reaction time;the first absorber structure has a thermal time constant; the ACcomponent has a maximum frequency; and the maximum frequency is basedupon the mechanical reaction time and the thermal time constant.
 6. TheMEMS bolometer system of claim 1, further comprising: a high pass filteroperatively connected to the absorber structure; and a demodulatoroperatively connected to an output of the high pass filter.
 7. A methodof operating a micro-electromechanical systems (MEMS) bolometer system,comprising: spacing a first absorber structure apart from a substrate bya first gap; selecting an AC frequency that compensates for noiseembedded in the system; generating a voltage with a DC component and anAC component, the AC component having the selected frequency; applyingthe voltage to a first electrode supported by the substrate and spacedapart from the first absorber structure to generate a first modulatedelectrostatic force on the first absorber structure using a firstelectrode; and modulating a minimum height of the first gap above thesubstrate using the first modulated electrostatic force.
 8. The methodof claim 7, wherein spacing the first absorber structure apart from thesubstrate by the first gap comprises: selecting a distance associatedwith a local maximum or minimum of a mean normalized absorption of thefirst absorber structure for a wavelength of interest; and spacing thefirst absorber structure apart from the substrate by the selecteddistance.
 9. The method of claim 8, wherein modulating the minimumheight of the first gap comprises: using the DC component to bias thefirst absorber structure from a first location associated with theselected distance to a second location closer to the substrate such thata first variation of the mean normalized absorption of the firstabsorber structure for a given change in the minimum height of the firstgap at the second location is greater than a second variation of themean normalized absorption of the first absorber structure for the givenchange in the minimum height of the first gap at the first location. 10.The method of claim 9, wherein spacing the first absorber structureapart from the substrate by the first gap comprises: spacing the firstabsorber structure apart from a mirror supported by the substrate at alocation aligned with the mirror.
 11. The method of claim 10, furthercomprising: positioning the mirror at a location between the firstelectrode and the at least a portion of the first absorber structure.12. The method of claim 9, wherein the first electrode comprises amirror supported by the substrate at a location aligned with at least aportion of the first absorber structure.
 13. The method of claim 7,further comprising: spacing a second absorber structure apart from thesubstrate by a second gap; generating a second modulated electrostaticforce on the second absorber structure using a second electrodesupported by the substrate and spaced apart from the second absorberstructure, the generating using the DC component and a further ACcomponent that is phase shifted by 180 degrees relative to the ACcomponent; and modulating a minimum height of the second gap above thesubstrate using the second modulated electrostatic force.
 14. The methodof claim 7, wherein: the first absorber structure has a mechanicalreaction time; the first absorber structure has a thermal time constant;the AC component has a maximum frequency; and the maximum frequency isbased upon the mechanical reaction time and the thermal time constant.15. The method of claim 7, further comprising: isolating a signalindicative of radiation absorbed by the first absorber structure from avoltage in the first absorber structure via a high pass filteroperatively connected to the absorber structure; and demodulating thesignal via a demodulator operatively connected to an output of thefilter.